The present invention relates generally to electromagnetic door locking devices and particularly to shear locks having improved features which ensure that the lock operates rapidly and positively during both locking and unlocking operations.
Various types of electromagnetic door locks are known. One type commonly referred to as a shear lock comprises an electromagnet mounted to or in a frame defining a doorway. An armature is movably mounted to travel with a door as the door moves in the doorway between an "open" and "closed" position. When the door is in the closed position, the armature is positioned in spaced relation from the electromagnet, but is mounted to or in the door such that when power is applied to the electromagnet the armature responds to the magnetic field and becomes engaged on an adjacent surface of the electromagnet.
Various styles and types of shoulder means such as ledges, tangs, and tabs have been employed to provide some physical interrelationship between the face of the electromagnet and the armature so as to enhance the lock's resistance to a shearing movement which would result from any attempt to open the door while power was applied to the electromagnet. On occasion, the various styles of shoulder means have provided sufficient mechanical interaction that, upon the removal of power from the electromagnet, the armature has failed to release from the face of the electromagnet. Such failure, to release can also occur due to residual magnetic fields remaining after the removal of Power from the electromagnet, or other causes.
It has been recognized that to insure proper disengagement between the electromagnet and the armature when power is removed from the electromagnet, some biasing means can be provided. Particularly where the armature is mounted in or on the top of the door, this biasing means acts in addition to any gravitational action on the armature itself to enhance the disengagement of the armature and electromagnet upon removal of power from the electromagnet. Examples of prior art devices which include such biasing means are found in U.S. Pat. Nos. 5,016,929 and 5,033,779.
The additional disengagement force provided by
the biasing means must be compensated for during the locking or engaging operation between the electromagnet and the armature. While it would seem to be possible to provide electromagnets of substantially increased size and field strength so as to overcome the additional force provided by the biasing means, such an electromagnet can be significantly more expensive to make and proportionally more difficult to properly install.
It would therefore be advantageous to provide and electromagnetic door locking assembly which includes appropriate biasing to insure positive unlocking action between the armature and electromagnet and to provide appropriate means to insure rapid positive locking operation despite the presence of the biasing means.